A fuel cell system using a fuel cell, such as a polymer electrolyte fuel cell or a phosphoric-acid fuel cell, configured to generate electric power by a reaction between hydrogen and oxygen is constituted by: a hydrogen generator configured to generate a fuel gas containing a large amount of hydrogen by a steam-reforming reaction from a hydrocarbon material, such as a city gas, propane, or kerosene; an air supply device configured to supply air as an oxidizing gas; and an electric power converter configured to convert DC power, generated by the fuel cell, into AC power. In some fuel cell systems, the electric power converter converts the DC power, generated by the fuel cell, into the AC power to supply the AC power to home electric power loads and interconnects an electric power system. With this, when the amount of electric power generated by the fuel cell system is smaller than a home electric power demand, the shortfall is compensated by the electric power system (commercial system). In contrast, when the amount of electric power generated by the fuel cell system is too large, the excess flows to the electric power system as a reverse power flow.
A method for operating such fuel cell system includes: a start-up step of increasing the temperature of the hydrogen generator up to a temperature suitable for the reaction (for example, PTL 1); an electric power generating step of, when the temperatures of respective portions of the hydrogen generator have reached predetermined temperatures, supplying the fuel gas from the hydrogen generator and the air from the air supply device to the fuel cell to obtain the electric power; and a stop step of terminating the electric power generation and performing a cooling or internal treatment such that the fuel cell and the hydrogen generator can be preserved till the next electric power generation.
PTL 1: Japanese Patent No. 3415086